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United States Patent |
5,687,674
|
Johanntgen
,   et al.
|
November 18, 1997
|
Steam power plant for generating electric power
Abstract
A steam power plant for generating electric power has a fossil-fuelled
boiler, a water-steam cycle for generating high-tension, superheated steam
for a steam turbine, an economizer to transmit heat from flue gas to the
feed-water, an air preheater to transmit flue gas heat to fresh air and
devices for removing dust, sulphur and possibly nitrogen from the flue
gases. In order to optimize heat exchange in the air preheater during
operation and reduce the heat loses during start-up of the steam power
plant, a heat exchanger system is provided with sections through which
recirculated air and a heat vehicle medium flow, in which the section
carrying the air is connected on the intake side to the fresh-air outlet
of the air preheater and on the outlet side to the fresh-air intake of the
air preheater.
Inventors:
|
Johanntgen; Uwe (Wadgassen, DE);
Marx; Franz Josef (St. Wendel, DE)
|
Assignee:
|
Saarbergwerke Aktiengesellschaft (Saarbrucken, DE)
|
Appl. No.:
|
367163 |
Filed:
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February 3, 1995 |
PCT Filed:
|
May 9, 1994
|
PCT NO:
|
PCT/DE94/00530
|
371 Date:
|
February 3, 1995
|
102(e) Date:
|
February 3, 1995
|
PCT PUB.NO.:
|
WO94/27089 |
PCT PUB. Date:
|
November 24, 1994 |
Foreign Application Priority Data
| May 10, 1993[DE] | 43 15 482.4 |
| Oct 15, 1993[DE] | 43 35 216.2 |
Current U.S. Class: |
122/1A; 110/234; 122/1C; 122/420; 122/421; 122/DIG.7 |
Intern'l Class: |
E22B 033/00 |
Field of Search: |
110/302,234,348
122/1 C,1 A,406.5,DIG. 7,420,421
|
References Cited
U.S. Patent Documents
4319458 | Mar., 1982 | Berkley | 60/693.
|
5293841 | Mar., 1994 | Suhr et al. | 122/1.
|
Primary Examiner: Sollecito; John M.
Assistant Examiner: Gravini; Steve
Attorney, Agent or Firm: Wray; James Creighton
Claims
We claim:
1. A steam power plant for generating electric power has a fossil-fuelled
boiler, a water-steam cycle for generating high-tension superheated steam
for a steam turbine, an economizer to transmit heat from flue gas to the
feed-water, an air preheater to transmit flue gas heat to fresh air, and
devices for removing dust, sulphur and possibly nitrogen from the flue gas
and is characterized in that it has a first heat exchanger system (16, 17)
with a section through which recirculated air and a heat vehicle medium
flow, whereby the section carrying the air is connected on the intake side
(14) to the fresh-air outlet of the air preheater (4) and on the outlet
side (13) to the fresh-air intake of the air preheater (4).
2. The steam power plant in accordance with claim 1 is characterized in
that it has a second heat exchanger system (6, 11, 12) which transmits at
least a part of the residual heat, which is still contained in the flue
gas before the desulphurization process, to the fresh air.
3. The steam power plant in accordance with claim 1 is characterized in
that the recirculation air stream of the first heat exchanger system (16,
17) can be adjusted (18).
4. The steam power plant in accordance with claim 1 is characterized in
that the heat carrier fluid in the first heat exchanger system (16, 17) is
feed-water from the water-steam cycle.
5. The steam power plant in accordance with claim 4 is characterized in
that the heat in the first heat exchanger system (16, 17) which is to be
removed for the most part can be transmitted to high pressure feed-water
(16) and the remainder to low pressure feed-water (17) of the water-steam
cycle.
6. The steam power plant in accordance with claim 2 is characterized in
that the second heat exchanger system (6, 11, 12) is a closed system with
water as a heat vehicle.
7. The steam power plant in accordance with claim 1 is characterized in
that the fresh air can be heated through heat exchange with draw-off steam
from the water-steam cycle of the power plant before it enters the air
preheater.
8. The steam power plant in accordance with claim 1 is characterized in
that the first heat exchanger system is a start-up heat exchanger (35),
whereby the section which carries the air of the start-up heat exchanger
(35) is connected on the intake side to the fresh-air outlet of the air
preheater (24) and on the outlet side (37) to the fresh-air intake of the
air preheater (24).
9. The steam power plant in accordance with claim 8 is characterized in
that the heat vehicle medium inside the start-up heat exchanger (35) is
hot water from the feed-water container (27) of the water-steam cycle of
the steam power plant.
10. The steam water plant in accordance with claim 9 is characterized in
that the feed-water can be further heated in a heat exchanger (49) which
is heated with condensed steam before it is cooled inside the start-up
heat exchanger (35).
11. The steam power plant in accordance with claim 10 is characterized in
that the cooled feed-water and the condensed steam can be fed (25, 51)
into the feed-water container (27).
12. The steam power plant in accordance with claim 8 is characterized in
that at least a part of the heat vehicle medium for the start-up heat
exchanger (35) can be used for increasing the feed-water temperature
before economizer (21).
13. The steam power plant in accordance with claim 12 is characterized in
that the heat vehicle medium is hot feed-water, which can be mixed with
the feed-water, which is fed (33) into the economizer (21) after it is
heated (49) again through heat exchange with condensed steam.
Description
BACKGROUND OF THE INVENTION
The invention describes a steam power plant for generating electric power
which has a fossil-fuelled boiler, a water-steam cycle for generating
high-tension, superheated steam for a steam turbine, an economizer to
transmit flue gas heat to the feed-water, an air preheater to transmit
flue gas heat to fresh air and devices for removing dust, sulphur and
possibly nitrogen from the flue gases.
In steam power plants which are operated with fossil fuels, i.e. gaseous,
fluid or solid fuels, the heat flux capacities (mass flow rate x specific
heat capacity) of the flue gas which is to be cooled and the fresh air
which is to be heated (combustion air) are different so that there is a
temperature difference of up to 90.degree. C. at the cold end of the heat
exchanger with a customary temperature difference between flue gas and
fresh air of approximately 30.degree. C. at the warm end of the air
preheater. These high differences in temperature result in corresponding
energy losses and have a corresponding negative effect on the overall
efficiency of the power plant.
Another disadvantage is that in power plants which are not equipped to
further use the residual heat which is still at a relatively high
temperature level in the flue gas which leaves the air preheater, be it
through reheating the cleaned flue gas before it enters the chimney or be
it through decoupling the heat for long-distance energy use, the residual
heat is destroyed in the flue gas desulphurizing plant. The result is
another decrease in the power plant's overall efficiency.
Another factor is that the start-up process in the known steam power plants
described in the introduction is not advantageous. For example, before
coal can be burnt in a coal-fuelled plant, considerable amounts of
expensive auxiliary combustibles such as oil or gas must be burned in the
boiler before the coal can be burnt until the parts of the plant--for
example the mills for the lignite pulverizer dryer of the coal, the
catalytic nitrogen removal reactor and the air preheater with its large
regenerative heat accumulator masses, which must be heated with the help
of the flue gas heat--reach their required minimum operating temperatures.
Furthermore, the steam which is produced during the start-up but also
during the shut down phase generally is precipitated in the capacitor of
the steam power plant without utilizing the heat.
The invention was charged with reducing the loss of energy in a power plant
as described in the introduction, to better utilize the heat of the flue
gas and to make the start-up process more economical overall by using less
oil or gas and by better utilizing the steam which is generated during the
start-up phase.
SUMMARY OF THE INVENTION
This task is solved in accordance with the invention through a steam power
plant which is characterized in that it has a first heat exchanger system
with sections through which recirculated air and a heat vehicle medium
flow and in which the section carrying the air is connected on the intake
side to the fresh-air outlet of the air preheater and on the outlet side
to the fresh-air intake of the air preheater.
Compared to the state of the art, the measures proposed in accordance with
the invention result in a clear reduction of energy losses in the air
preheater, in a considerably improved utilization of the heat which is
contained in the flue gas, and therefore in a marked increase of the
overall efficiency of such a steam power plant.
The recirculation air stream of the first heat exchanger system which is
superimposed to the fresh air in the area of the air preheater makes it
possible to almost completely align the heat flux capacities in the two
heat exchanger sections of the air preheater. The result is that there are
low temperature differences at the warm, as well as at the cold end, and
correspondingly reduced energy losses. Some of the flue gas heat which is
released in the air preheater which was transmitted to relatively cold
fresh air in the customary design is now directly coupled into the
water-steam cycle of the power plant at a higher temperature level via the
recirculation air stream and the first heat exchanger system.
Since the recirculation air stream in accordance with the invention makes
it possible to regulate the equilibrium of the heat flux capacities
between the two sections in the air preheater, the fresh air can be
preheated before it is introduced into the air preheater, i.e. the residue
heat which is contained in the flue gas before it is desulphurized can be
used in the entire process due to the transmission of the fresh air into
the second heater exchange system in accordance with the invention. The
result therefore is a further improvement of the overall efficiency of the
power plant.
The invention is especially advantageous for power plants in which the
desulphurized cold flue gases are introduced directly into the cooling
tower of the power plant and are released into the atmosphere together
with the cooling air and in which it is possible to decouple the residue
heat of the flue gas for long-distance energy uses or other uses. In these
cases the design in accordance with the invention makes it possible to
reintroduce the entire residual heat which is still contained in the flue
gas into the power plant cycle under thermodynamically favorable
conditions.
In power plants which use the residual flue gas heat either for
long-distance energy purposes or for heating the desulphurized flue gases
which are to be released into the atmosphere via a chimney, the overall
efficiency can be improved with the help of another characteristic of the
invention by preheating the fresh air before it enters the air preheater,
in a steam-air-preheater by transmitting the low temperature heat from the
water-steam cycle.
The design in accordance with the invention generally makes it possible to
bring the low temperature heat, which is produced in the power plant, to a
higher temperature level by transmitting it to the fresh air, by coupling
it into the air preheater and by reintroducing it into the water-steam
cycle.
Important advantages with regard to the start-up phase of a steam power
plant can be achieved if--as intended by the invention--the first heat
exchanger system is used as a start-up heat exchanger. This makes it
possible to preheat the regenerative heat accumulations of the air
preheater already before the start-up phase of the steam power plant
begins, i.e. already before the burners in the boilers are ignited.
For this purpose heat is transmitted to the recirculation air which is in
the cycle between the still cold air preheater and the start-up heat
exchanger via any heat vehicle in the start-up heat exchanger, and the
regenerative heat accumulations of the air preheater are heated in the
process.
Due to the preheating of the fact that the air preheater is preheated, the
fresh combustion air, which is introduced into the boiler via the air
preheater during the consequent start-up of the steam power plant, is
heated correspondingly. The results are advantageous effects during
cold-start with boiler surfaces that are already cold, as well as during
hot start with boiler surfaces that are still hot. The plant heats up
faster during a cold start, i.e. less oil or gas is needed in the boiler
while there is a minimal super-cooling due to inflowing cold combustion
air during a hot start.
The advantage with coal-fuelled steam power plants--and this constitutes
the most important application of the invention--is that by preheating the
air preheater, the minimum temperatures following the air preheater, which
are required for the start-up of the coal heat lignite pulverizer dryer,
can be reached quicker. The result is that it is possible to switch faster
from the oil or gas fuelling, which is customary during the start-up, to
the normal coal fuelling. Another advantage is that the operating
temperatures needed for the nitrogen removal reactor can be reached faster
while the efficiency of the nitrogen removal, and therefore the effects on
the environment, are influenced accordingly.
Another advantage which results from preheating the air preheater is that
when flue gas which is not yet desulphurized is first introduced into the
air preheater, a strong cooling with a corresponding dew point is avoided
which results in a correspondingly reduced corrosion inside the air
preheater, as well as in any subsequent parts of the facility, e.g. the
electrostatic filter.
As already stated, the start-up heat, which is coupled into the air
preheater via the start-up heat exchanger, can be from any source. It can
be, for example, the heat from another steam power plant which is located
at the same site, or it can be the waste heat of any other industrial
plant.
For reasons of practicality, another characteristic of the invention is
that the heat vehicle is feed-water from a feed-water container of the
water-steam cycle, which is already heated to temperature by the start-up
steam from the steam power plant, from neighboring plants or from separate
boilers.
If necessary, the temperature of the used feed-water can be further
increased in another heat exchanger through heat exchange with the
condensed start-up steam before it is cooled in the start-up heat
exchanger.
Further explanations concerning the invention can be taken from the
examples which are shown schematically in the figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an example of a circuit lay-out for decreasing heat losses
during the operation of a steam power plant.
FIG. 2 shows an example of a circuit lay-out for decreasing heat losses
during the start-up phase of a steam power plant.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In accordance with FIG. 1, hot flue gas from a steam generator of a
coal-fuelled power plant is transmitted to a nitrogen removal plant 2 and
then to an air preheater 3 with a temperature of approximately 380.degree.
C. via a line 1. Inside the air preheater 3 the flue gas is cooled to
130.degree. C. due to the heat exchange with air. After it has passed
through an electrostatic filter 4 and an induced draught ventilator 5, the
flue gas is further cooled from approximately 130.degree. C. to
approximately 80.degree.-90.degree. C. in a heat exchanger 6 of a second
heat exchanger system which consists of heat exchangers 6 and 11 as well
as a cycle water system 12. Then the cooled flue gas is fed into a flue
gas desulphurization plant 8 via a line 7 and then is released into the
atmosphere together with the cooling air via the cooling tower of the
power plant which is not shown.
The combustion air which is needed inside the steam generator is supplied
to the power plant via a line 9 and a ventilator 10 and at first is
preheated to a temperature of approximately 70-.degree.80.degree. C. in a
heat exchanger 11. The heat needed for the preheating process is
transmitted via a closed cycle water system 12 by the heat exchanger 6
into the heat exchanger 11.
At a mixing point recirculating air--whose temperature and mass flux is
such that there is an approximate heat flux equilibrium inside the air
preheater 3, i.e. the desired small temperature differences between the
flue gas and the air are now present at the cold as well as the warm end
of the air preheater--is added to the fresh air which is preheated in the
heat exchanger 11.
After the recirculation air stream has passed the air preheater, it is
again separated from the fresh air stream at a separation point 14. While
the fresh air is supplied to the fuelling of the steam generator via a
line 15 and at a temperature of 350.degree. C., the recirculation air is
cooled again in a first heat exchanger system in a heat exchanger 16 by
exchanging heat with high pressure feed-water and, if necessary, in a
second heat exchanger 17 by exchanging heat with low-pressure feed-water
and then is transported back to the mixing point via an adjustable
ventilator 18.
FIG. 2 shows schematic sections of a circuit of a coal-fuelled steam power
plant. The hot flue gas is supplied from an economizer 21 of the steam
generator power plant via a line 22 to a catalytic nitrogen removal
reactor 23 and finally to an air preheater 24. Inside the economizer 21
the flue gas is cooled to the optimal operating temperature of the
nitrogen removal reactor 23 of approximately 350.degree.-380.degree. C.
through heat exchange with feed-water. It is cooled to approximately
130.degree. C. through heat exchange with fresh combustion air in the
subsequent air preheater 24. After it is cooled down, dust or sulphur are
removed from the flue gas in devices which are not shown, and then the
flue gas is released into the atmosphere together with cooling air via a
cooling tower which is also not shown.
The combustion air which the plant needs for the boiler is supplied via a
line 25, is heated to approximately 350.degree. C. in the air preheater
24, and then is supplied to the fuelling or lignite pulverization dryer
via a line 26.
The shown section of the water-steam cycle of the plant shows a feed-water
container 27 in which the condensate, which is supplied via line 28, is
heated by the steam from line 29. The heated water (feed-water) is removed
from the feed-water container 27 via a line 30 which is pumped up to
approximately 250-300 bar in a jetting pump and then is preheated to a
temperature of approximately 250.degree.-300.degree. C. in a customary
high-pressure preheater 32. The preheated feed-water flows into the
economizer 21 via a line 33 in which is heated again through heat exchange
with hot flue gas. The feed-water then is brought into the other heat
exchanger system of the boiler via a line 34 and there evaporates or is
superheated to the starting temperature of the steam turbine of
approximately 530.degree.-580.degree. C.
After the pressure in the turbine is relieved, the steam is condensed and
is again brought to the feed-water container 27 via line 28.
The above description of the system of a steam power plant is based on
normal operations for full loads or partial loads.
The invention intends to make the start-up process of such a power plant
more economical. For this purpose the invention calls for a start-up heat
exchanger 35 with a section through which recirculated air flows and which
is connected on the intake side to the fresh air outlet of the air
preheater 24 via a line 36 and on the outlet side to the fresh air intake
of the air preheater 24 via a line 37 and a ventilator 38.
Before or during the start-up process of the power plant the recirculated
air which is between the air preheater 24 and the start-up heat exchanger
35 is heated inside the start-up heat exchanger 35 and is cooled again in
the air preheater 24, whereby the regenerative heat accumulations of the
air preheater heat up. This preheating process on one hand causes the flue
gas, which is produced at the beginning of the start-up process, to not be
cooled as much inside the air preheater so that the dew point underflow
and related corrosion damage inside the air preheater and any subsequent
plant devices can be prevented. Furthermore, it is possible to transmit
additional heat to the combustion air via the recirculated air during the
start-up process. This in turn makes it possible to reach the required
temperature for starting the lignite pulverization dryer and therefore the
temperature for starting the coal burner of the boiler quicker. The result
is that now the auxiliary burners, which are operated with expensive oil
or gas, can be shut off earlier, and the flue gas side can reach the
operating temperatures quicker (corrosion).
In accordance with the example shown in the figure, the recirculated air of
the heat exchanger 35 is heated through heat exchange with hot feed-water
which is drawn off in the feed-water container 27 by injecting start-up
steam which is transported via a line 29 and via lines 39, 40, 41 with the
correspondingly opened valves 42 and 43, is cooled in the heat exchanger
35, and then is returned back into the feed-water container 27 via lines
44 and 45 and the open valve 46.
If necessary, the feed-water can be heated again before it enters the heat
exchanger 35 by supplying at least a partial stream of the heated
feed-water in line 39 via a now open valve 47 and a line 48 into another
heat exchanger 49. The heat exchanger 49 is heated with the help of the
condensed start-up steam from a source 50 which then is supplied into the
feed-water container 27 via a line 51.
The feed-water stream, which is further heated in the heat exchanger 49, is
first supplied into line 41 and then into the heat exchanger 35 via a line
52 and an open valve 53.
In accordance with another model of the invention, not only the air
preheater 24 but also the economizer 21 is supplied with additional heat
during the start-up process of the steam power plant. For this purpose at
least a partial stream of the feed-water which was heated in the heat
exchanger 49 is fed into line 33 via an open valve 54 and lines 44 and 55
and there is mixed with the feed-water from the high pressure preheater
32. This is shown in the example in the figure. Now it is possible to use
the start-up heat. Due to the increase in temperature of the feed-water,
the flue gas inside the economizer does not cool as much during the
start-up process so that the minimum temperature for starting the
subsequent nitrogen removal reactor 23 can be reached faster.
The heat transfer onto the storage accumulation of the air preheater 24 or
the additional heat transfer onto the combustion air or the feed-water
makes it possible to use the heat of the steam which is produced during
the start-up phase. Storing the steam in the air preheater surfaces also
makes it possible to use the steam which is produced in the course of the
start-up phase.
The proposed coupling of the additional heat into the economizer proved to
be beneficial not only during the start-up process of the power plant, but
it can also be used for supporting the flue gas temperature, and therefore
for maintaining the optimal operating temperature of the nitrogen removal
reactor 23 during light load operation of the power plant.
By varying the amount of the recirculation air via the ventilator 38 as
well as the transmitted heat amount inside the heat exchanger 35, the flue
gas temperature after the air preheater 24 can be maintained at a constant
level in the entire load area; this means that even during full load
operation it is possible to obtain an optimally low flue gas temperature
without having to accept any low temperatures and therefore corrosion
during partial load, for example.
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